Method of treating magnesium



Feb. .11, 1941. c. r; NELSON METHODy OF TREATING MAGNESIU Filed Feb. 25, 1959 2 Sheets-Sheet 1 l 0 RM ys o/ Y MM; M E Q R W1. im uw A M Patented Feb. 11, 1941 UNITED STATES METHOD F TBEATING MAGNESIUM Charles E. Nelson, Midland, Mich., assigner to The Dow Chemical Company, Midland, Mich., a corporation of Michigan Application February 23, 1939, Serial N0. 257,924

Claiml.

This invention relates to the treatment oi magnesium. It more particularly concerns a method of treating commercial magnesium whereby it is rendered highly pure or the magnesium in magnesium scrap. such as castings, alloys and the like, can be recovered in a highly pure state.

I have found that the presence of certain impurities in magnesium, even though these be m present in very limited amounts, greatly accelerates the rate at which magnesium corrodes. For example, it is recognized that the presence of a small amount of iron in magnesium greatly accelerates the rate at which magnesium corl rodes and may thus render it unsuitable for many commercial uses. The impurities nickel and copper atleet the rate at which magnesium metal corrodes similarly.

To remove these impurities ordinarily present in the magnesium produced by present practices, it has heretofore been proposed to subject the magnesium to sublimation or distillation. However, I have observed that unless the metal being puriiled is subjected to repeated sublimation or 2B distillation, as the case may be. the impurities are not generally removed to the extent that they no longer greatly accelerate the rate at which magnesium corrodes or otherwise adversely affect the properties of the metal. In some instances, even by repeated distillations or sublimations, the metal cannot be rendered satisfactorily pure.

It is, therefore, the principal object of the invention to provide a method oi rendering magnesium so highly pure that the impurities no longer have substantially any adverse eiiect as regards the rate at which magnesium corrodes. Another object of the invention is to provide a method whereby magnesium can be rendered I pure in a single operation. Other objects and advantages will be apparent as the description proceeds.

I have discovered that if the magnesium to be puriiled is caused to vaporize in the presence of a metal selected from the group of metals consisting of lead, tin, bismuth, calcium, silicon, and

antimony, the impurities can be removed to the'y extent that the rate at which ordinary commer. cial magnesium corrodes is greatly reduced. The invention then resides in themethod hereinafter fully described and particularly pointed out in the claims, the accompanying drawing and following description setting iorth in detail a mode of carrying out the invention. such mode as illustrating, however. but one oi' various ways in (Cl. 'i5-67) which the principle of the invention may be used.

In said drawing:

Fig. 1 is a sectional view in elevation of the apparatus.

Fig. 2 is a top plan View of said apparatus.

Fig. 3 is a view ofthe apparatus illustrated in Allig. 1 showing a slightly modified structure.

In the drawing an upright cylindrical iron retort i is shown closed at the bottom i1 and prom vided at the top end with a flange I, to which is attached a gasket sealed head 3 by means of suitable bolts I. Condenser 5, in the form of a section or iron pipe closed at its lower end ris attached centrally to the interior side of head u 8 and extends into the retort. Exteriorly of the retort a cooling water inlet I and water outlet 'I communicate with the interior of the condenser 5. A mass of purified condensed magnesium I is shown clinging to the condenser l. 0 A vacuum connection 8 communicates through head I with the interior of the retort and provides a means whereby the retort may be connected to a vacuum pump (notshown) and held under reduced pressure. A cooling coil I0 inti- 25 mately attached to head 3 (as by welding) encircles the head directly above gasket Il and serves to prevent the gasket from -becoming overheated. Interiorly oi the retort a.A removable iron charge container I2 partially tllled with a80 solid charge I3 consisting of a quantity o! impure magnesium and a suitable metal added to ei'iect the purification4 of the magnesium rests on the bottom ot the retort. As a means of heating the retort, eiectical leads I4 are connected 35 to resistance wire I5 which encircles the lower portion o! the retort. A suitable insulating material II surrounds the retort and serves to protect it from excessive heat loss due to radiation.

In the view illustrated in Fig. 3 the apparatus 40 Aof Fig. l has been modiiled by suspending a sha1- low pertorate metal basket Il over charge container I2. The basket I I is suspended from the charge container I2 by meansof ange Il which engages the top portion of charge container i2. 45 A periorate cover 2l is Ashown seated on top oi the basket Il and acts to confine the filter bed. Basket I8 serves as a container for a illtering material designated generally by numeral 2i in case such is used. 5

In carrying out the method of the invention, the iron container l2 is filled with a charge I8 consisting of impure magnesium and the added purifying metal and lowered into the retort through the open end. The retort is then Aclosed up by bolting flanged head 3 in place. Cooling water is then started through lines 8 and 'i and through coil I while vacuum is applied to exhaust the interior of the retort of gases through line 9. When the desired degree of vacuum is obtained, electric cunent is supplied to the resistance Wire i through electrical leads I4 to heat the retort, care being taken in the meantime to maintain the vacuum. The impure magnesium as well as the added metal rapidly melts and, as magnesium vapor forms, it collects and condenses on the cold surface of condenser 5 where it adheres in the form of compact crystals. After such a time has elapsed as will indicate that substantially all of the magnesium has been distilled off, the current may be shut off, the vacuum broken, and the flange head detached from the retort. The magnesium crystals can then be stripped from the condenser. Additional impure magnesium may be immediately added to the charge container without removing the residue therefrom and the process of purifying metal continued.

As afore-mentioned, the metals suitable for use are the metals selected from the group of metals consisting of lead, tin, bismuth, calcium, silicon and antimony. If desired, mixtures of these metals may be suitably employed. For

example, a particularly effective mixture consists of tin and lead. In general the amount oi such metal to employ to eiect the removal of undesirable impurities is from l0 to 50 per cent or more by weight, based upon the weight of the impure magnesium, and preferably between 25 to 35 per cent. It is unnecessary to remove the residue, consisting mainly of the added metal, from the charge container after every run, since in practice it has been found that as many as 20 or more charges of impure magnesium may be purified with but a single addition of the purifying metal.

it is usually desirable to operate under relatively high vacuum, as for example, less than 2 millimeters of mercury, and preferably below 0.2 of a millimeter, since under such highly reduced pressure the purified magnesium condenses out in the form of a dense compact mass rather than in the form of long needle-like crystals. Condensation in a dense compact form is desirable because relatively little surface area is exposed to oxidation when the metal is later reliandled for remelting or the like, in comparison with the area exposed when a needle-like crystal structure is produced.V Operating at highly reduced pressure offers the additional advantage that the temperature required to effectively calry out the process is such that the apparatus may be constructed of readily available iron or steel Without any danger of destruction due to the high temperature necessary if the purifying operation is carried out at higher pressure. If desired, a suitable inert gas may be introduced into the retort to prevent den structive attack by air or other gases upon the purified metal, falthough under the preferred vacuum conditions such procedure is unnecessary.

The temperature to be employed in the distillation or sublimation may, of course, be varied quite widely, as for example between about 500 to 800 C., depending however upon the degree of vacuum employed. At 0.05 millimeter pressure of mercury a temperature of '700D C. has been found highly satisfactory. It is preferable that the magnesium be caused to vaporize at such a temperature that the added metal melts in the presence of the magnesium vapor.

The time required to purify a batch of metal also depends upon the amount of heat supplied to the metal as well as the pressure employed. In a retort such as illustrated, 16 inches in diameter, it -is possible to distill oil' from three to four. pounds of magnesium per hour by holding the temperature at about 700 C. and the pressure at 0.05 millimeter.

Although magnesium of a relatively high degree of purity and, therefore, highly resistant to corrosion can be produced in the manner above described I have made the further discovery that by using the present process in conjunction with a filtering operation, such as is disclosed in my co-pending applications Serial Nos. 257,923, illed February 23, 1939 and 257,871,` filed February 23, 1939, magnesium of even a higher degree of purity can be obtained with a consistency that is not possible when employing either of the methods alone. In the above said co-pending applications there is disclosed and claimed methods of purifying magnesium either by causing the magnesium vapor to pas through a porous, granular, or fibrous filter bed of a refractory selected from the group of re- Iractories consisting of the oxides and silicates located above manganese in the electromotive series of metals, or by causing the vapor to pass through a porous filter bed of a granular salt selected from the group of salts consisting of the halides of the alkali and alkaline earth metals. A purifying operation, wherein the magnesium vapor is passed through a filter bed of a refractory or a halide salt, as well as being vaporized in the presence of one of the metals, such as lead or the like, in the above described manner, is carried out by suspending the shallow perfor-ate metal basket filled with a filtering medium over the charge container in such a manner that the magnesium vapor filters through the prepared bed. The depth of the filter bed so prepared is found to operate most effectively if it is between from 1/2 to 2 inches. The temperature and pressure conditions under which the combined processes may be operated satisfactorily are generally the same as for either process alone. In general, when employing a Yfilter in combination with a metal, a smaller amount of the purifying metal gives satisfactory results. For example, when employing a metal alone, the preferred amount is from 25 to 35 per cent by weight. When employing the combined processes, from 15 to 25 per cent of the metal is generally preferable. A generally desirable combination to employ, because of the high degree oi.' purity of the product obtained, is from l5 to 20 per cent by weight of lead in combination with a filter composed of rock wool.

Other modes of applying the principle o! my invention may be employed instead of those explained, change being made as regards the method herein disclosed, provided the step or steps stated by any of the following claims or the equivalent of such stated step or steps be employed.

I therefore particularly point out and distinctly claim as my invention:

1. The method of producing substantially pure magnesium comprising bringing impure magnesium into direct contact with from 10 to 50 per cent by weight of a metal selected from the group of metals consisting of lead, tin, calcium, bismuth, antimony and silicon, applying heat so as to cause the metals to alloy, and thereafter continuing the application of heat so as to cause the magnesium to vaporlze from the so-alloyed mixture.

2. The method oi producing substantially pure magnesium comprising bringing impure magnes ium into direct contact with from 10m 50 per cent by weight of a metal selected from the group of metals consisting of lead, tin, calcium, bismuth, antimony and silicon, maintaining a vacuum on the metals while applying heat so as to cause the metals to alloy, and thereafter continuing the application of heat so as to cause the magnesium to vaporize from the so-alloyed mixture.

3. The method of producing substantially pure magnesium comprising bringing solid impure magnesium into direct contact with from 25 to 35 per cent by weight of a metal selected from the group of metals consisting of lead, tin, cal-A cium, bismuth, antimony and silicon, applying heat so as to cause the metals to alloy, continuing the application of heat so as to cause the magnesium to vaporize from the so-ailoyed mixture while maintaining the metals under a pressure of less than 2 millimeters of mercury, and thereafter condensing the magnesium vapor.

4. The method of producing substantially pure magnesium comprising introducing impure magnesium into direct contact with from 10 to 50 per cent by weight of a metal selected from the group of metals consisting of lead, tin, calcium, bismuth, antimony and silicon. applying heat so as to cause the metals to alloy, continuing the application' of heat so as to cause the magnesium to vaporize, and thereafter passing the magnesium vapor through a illter bed of a solid granular salt selected from the group of salts consisting of the halides of the alkali and alkaline earth metals.

5. The method of producing substantially pure magnesium comprising bringing impure magnesium into direct contact with from 25 to 35 per cent by weight of a metal selected from the group of metals consisting of lead, tin, calcium, bismuth, antimony and silicon, applying heat so as to cause the metals to alloy, continuing theI appiication of heat so as to cause the magnesium to vaporize while maintaining the metals under highly reduced pressure, and thereafter passing the magnesium vapor through a filter bed of a solid salt selected from the group of salts consisting of the halides of the alkali and alkaline earth metals.

6. riihe method of producing substantially pure magnesium comprising heating an alloy consisting of magnesium and from 10 to 50 per cent by weight of at least one of the metals selected from the group of metals consisting of lead, tin, calcium, bismuth, antimony and silicon so as to cause the magnesium contained in the alloy to vaporize, passing the magnesium .vapor through a filter bed of a solid granular salt selected from the group of salts consisting of the halldes of the alkali and alkaline earth metals, and thereafter condensing the magnesium vapor.

.. CHARLES E. NELSON. 

